Whole-rock and mineral geochemistry of ODP Hole 176-735B gabbros@en

We report mineral chemistry, whole-rock major element compositions, and trace element analyses on Hole 735B samples drilled and selected during Leg 176. We discuss these data, together with Leg 176 shipboard data and Leg 118 sample data from the literature, in terms of primary igneous petrogenesis. Despite mineral compositional variation in a given sample, major constituent minerals in Hole 735B gabbroic rocks display good chemical equilibrium as shown by significant correlations among Mg# (= Mg/[Mg + Fe2+]) of olivine, clinopyroxene, and orthopyroxene and An (=Ca/[Ca + Na]) of plagioclase. This indicates that the mineral assemblages olivine + plagioclase in troctolite, plagioclase + clinopyroxene in gabbro, plagioclases + clinopyroxene + olivine in olivine gabbro, and plagioclase + clinopyroxene + olivine + orthopyroxene in gabbronorite, and so on, have all coprecipitated from their respective parental melts. Fe-Ti oxides (ilmenite and titanomagnetite), which are ubiquitous in most of these rocks, are not in chemical equilibrium with olivine, clinopyroxene, and plagioclase, but precipitated later at lower temperatures. Disseminated oxides in some samples may have precipitated from trapped Fe-Ti-rich melts. Oxides that concentrate along shear bands/zones may mark zones of melt coalescence/transport expelled from the cumulate sequence as a result of compaction or filter pressing. Bulk Hole 735B is of cumulate composition. The most primitive olivine, with Fo = 0.842, in Hole 735B suggests that the most primitive melt parental to Hole 735B lithologies must have Mg# 0.637, which is significantly less than Mg# = 0.714 of bulk Hole 735B. This suggests that a significant mass fraction of more evolved products is needed to balance the high Mg# of the bulk hole. Calculations show that 25%-45% of average Eastern Atlantis II Fracture Zone basalt is needed to combine with 55%-75% of bulk Hole 735B rocks to give a melt of Mg# 0.637, parental to the most primitive Hole 735B cumulate. On the other hand, the parental melt with Mg# 0.637 is far too evolved to be in equilibrium with residual mantle olivine of Fo > 0.89. Therefore, a significant mass fraction of more primitive cumulate (e.g., high Mg# dunite and troctolite) is yet to be sampled. This hidden cumulate could well be deep in the lower crust or simply in the mantle section. […]本研究报道了第176航次钻探选取的735B钻孔（Hole 735B）样品的矿物化学（mineral chemistry）、全岩主量元素组成（whole-rock major element compositions）及微量元素分析结果。我们结合本次航次的船载数据与已发表的第118航次样品数据，围绕原生火成岩成因（igneous petrogenesis）对这些数据展开讨论。尽管单一样品内部存在矿物成分变异，但735B钻孔辉长岩类岩石中的主要造岩矿物展现出良好的化学平衡特征：橄榄石（olivine）、单斜辉石（clinopyroxene）与斜方辉石（orthopyroxene）的镁指数（Mg# = Mg/[Mg + Fe²+]），以及斜长石（plagioclase）的钙长石分子比（An = Ca/[Ca + Na]）之间存在显著相关性。这表明，橄长岩（troctolite）中的橄榄石+斜长石组合、辉长岩（gabbro）中的斜长石+单斜辉石组合、橄榄辉长岩（olivine gabbro）中的斜长石+单斜辉石+橄榄石组合，以及辉长苏长岩（gabbronorite）中的斜长石+单斜辉石+橄榄石+斜方辉石组合等，均从各自的母熔体中共结晶析出。绝大多数这类岩石中普遍产出的铁钛氧化物（Fe-Ti oxides）包括钛铁矿（ilmenite）与钛磁铁矿（titanomagnetite），并未与橄榄石、单斜辉石及斜长石达到化学平衡，而是在更低温度下后期结晶形成的。部分样品中的浸染状氧化物可能源自被捕获的富铁钛熔体的结晶作用。沿剪切带/剪切区富集的氧化物则可能代表了因压实作用或滤压作用从堆晶岩（cumulate）序列中排出的熔体汇聚/运移区域。735B钻孔整体岩石具有堆晶岩组成特征。该钻孔中最原始的橄榄石（Fo值为0.842）指示，对应735B钻孔岩性的最原始母熔体的镁指数应为0.637，这显著低于全钻孔整体的镁指数（0.714）。这表明需要混入相当比例的演化程度更高的产物，才能平衡全钻孔整体的高镁指数。计算结果显示，需将25%~45%的东亚特兰蒂斯二世断裂带（Eastern Atlantis II Fracture Zone）平均玄武岩与55%~75%的735B钻孔整体岩石混合，才能得到镁指数为0.637的熔体——该熔体即为735B钻孔中最原始堆晶岩的母熔体。另一方面，该镁指数为0.637的母熔体演化程度过高，无法与Fo值大于0.89的残留地幔橄榄石（residual mantle olivine）达到平衡。因此，仍有相当比例的更原始堆晶岩（例如高镁指数纯橄岩（dunite）与橄长岩）尚未被采样。这类未被发现的堆晶岩可能位于下地壳深部，或是直接赋存于地幔剖面中。[...]